1. Field of the Invention
The present invention relates to a high density optical recording medium with a self-sustaining capability to diminish the spot size of an incident focused laser beam below a diffraction-limited size during recording and readout of information, and more particularly, to an optical recording medium in which purposely combined layers of medium materials can reduce additionally the spot size of a laser beam, once focused and irradiated on the recording medium through a far field optic system, to below a diffraction-limited size and maintain it during recording as well as readout and therefore result in a remarkable increase in storage density.
2. Description of the Background Art
A primary method to boost the recording density of an optical recording medium is to reduce the spot size of a laser beam in use for recording and reproducing information. When a laser beam of a wavelength λ with a Gaussian intensity profile is focused through the objective lens of a numerical aperture NA, it has a full width half maximum (FWHM) spot size of 0.59 λ/NA at diffraction limit, setting a limit in readout resolution to 2NA/λ in terms of a spatial frequency. Accordingly, in order to achieve a high density for an optical recording medium by use of a diffraction-limited focused light, it is necessary to reduce the wavelength λ or to increase the numerical aperture of the objective lens. However, optical recording technology has already proceeded to use of laser light with a shortest wavelength in the visible regime (λ=405 nm) as well as a high NA (=0.85), and a storage density is considered to have come close to the theoretical limit in this line of approach.
Numerous techniques have been proposed to overcome light diffraction limit and thereby to accomplish a high density recording and readout. Among them, super-resolution techniques make it possible to reproduce a high density information of a spatial frequency above 2NA/λ by use of a reduced beam spot relative to the one at diffraction limit, which derives from optical, thermo-structural or thermo-magnetic characteristics of materials of an optical recording medium. These techniques have a remarkable advantage over other techniques that a high density can be achieved beyond that of diffraction limit, yet on the basis of a far-field optic system of the existing optical recording devices.
Led by a U.S. Pat. No. 5,153,873 which discloses a super-resolution optical recording medium including a single layer nonlinear material, super-resolution techniques have been proposed utilizing various groups of materials such as thermochromic materials, photochromic materials, phase change materials, optical bistable materials, saturable-absorption materials, and higher-order nonlinear optical materials.
Each of these super-resolution techniques makes use of a single super-resolution layer in an optical recording medium to produce a laser beam spot smaller than the one at diffraction limit during readout, but mostly neglects the problem of a varying spot size of the transmitted light (hereafter transmitted light is used to refer to light transmitted through a super-resolution layer unless specified otherwise) with incident light power.
Accordingly, any of such techniques may not be applied successfully to a recordable or a rewritable type optical recording medium for which a reduced spot size is required during readout and recording as well in order to achieve a higher storage density than what is accessible with diffraction-limited light.
U.S. Pat. No. 5,420,846 discloses a method to overcome such a problem utilizing a single layer of a super-resolution material that exhibits a two-step change in optical transmittance with increasing light intensity. It should be of limited applicability, however, due to rare and severely imposing material characteristics required of a super-resolution layer.
Therefore, a method is called for that provides a reduced spot size below a diffraction-limited size during readout and recording as well while offering a much wider window of choice of materials for super-resolution.